MgB2 Thin Films on Metallic and Dielectric Substrates for Microwave Electronic and SRF applications D. E. Oates MIT Lincoln Laboratory, Lexington MA Y. D. Agassi Naval Surface Warfare Center, Carderock Division, Bethesda MD B. H. Moeckly and C. Yung STI Inc. Santa Barbara, CA G. Carpenter and F. Niu SVT Associates, Eden Prairie, MN The Lincoln Laboratory portion of this work is sponsored by the Naval Surface Warfare Center, Carderock Division under Air Force Contract #FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government.   This work was sponsored by the Defense Threat Reduction Agency, the Office of Naval Research, and the Naval Surface Warfare Center, Carderock Division

MgB2 Discovered to be superconducting in 2001 with a Tc = 39 K J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani and J. Akimitsu,“Superconductivity at 39K in Magnesium Diboride,” Nature 410, 63 (2001) Believed to be BCS superconductor Bulk and thin films No isostructural compounds show high Tc

Outline Introduction to MgB2 and Motivation Film deposition Measurements of surface impedance Physics of MgB2 from microwave measurements Summary

Motivation RF applications of MgB2 thin films TC = 40 K Operating T ≈ 20 K Low surface resistance Comparable to niobium at 4 K Long coherence length  = 4 – 8 nm Polycrystalline films with good RF properties Grain boundaries are not weak links High Critical Field Hc ≥ 1.5 T Good power handling Crystal structure AlB2 (Hexagonal) Stable, stoichiometric

Applications RF cavities coated with MgB2 for accelerator applications Improvement on niobium technology Films on metallic substrates High Q at high power HC → RF breakdown Passive microwave electronics Films on dielectric substrates Inexpensive polycrystalline substrates High Q at low to medium power Low-loss delay lines – dispersive delay lines Miniature filters Intermodulation distortion possible issue

Outline Introduction to MgB2 and Motivation Film deposition Measurements of surface impedance Physics of MgB2 from microwave measurements Summary

Reactive Evaporation Film Deposition Solves MgB2 Film-growth Difficulties: Mg Volatility, Oxidation Rotating blackbody heater Advantages: Localized Source of High-Pressure Mg Vapor Different Mg and Substrate Temperatures Films: TC  39K, TC  1K, Resistivity(Tc)  2Ω-cm 4” Wafers, scale up possible RMS roughness = 4.4 nm Schematic view of the film growth by reactive evaporation. Using the growth technique of reactive evaporation in which a localized source of Mg vapor is maintained near the substrate by using a rotating pocket heater, we have been able to grow films completely in situ. B. H. Moeckly et al., Supercond. Sci. Technol. 19, L21 (2006)

MgB2 Thin-Film Properties AFM surface scan 0.5-m film on sapphire 40 >Tc > 39.5 Low resistivity Sharp transition 500-nm MgB2 film on Nb RMS surface roughness = 3.0 nm AFM of the polished niobium substrate on the left and the deposited MgB2 film on the right. Roughness increases slightly but the result is quite smooth.

Atomic Layer Deposition of MgB2 Joint Program with SVT Associates Developing ALD process and system for MgB2 deposition B2H6 and Mg(CpEt)2, or Mg(thd)2 Plasma enhanced Conformal coating method Needed for cavity coating Progress to date: thin films of MgBx Still developing the process parameters

Outline Introduction to MgB2 and Motivation Film deposition Measurements of surface impedance Small samples 1 cm x 1cm 2-inch diameter wafers Physics of MgB2 from microwave measurements Summary

Stripline Resonator for Measurements on Dielectric Substrates IMD measurement Stripline resonator Resonator f1 f2 + Spectrum Analyzer Patterned center line Capacitive 150 m coupling Input Spectrum f1 f2 Frequency Power Output Spectrum f1 f2 2f1 - f2 2f2- f1 Fundamental The left panel shows the stripline resonator. The films are patterned and packaged as stripline resonators for the characterization of the microwave properties. This device is used for measurement of ZS(Irf,T,f), intermodulation distortion, and 3rd harmonic generation. The right panel is the IMD system. Shown at the top is the block diagram for the measurements of intermodulation distortion. The second line shows the input and output spectra. We apply two closely spaced tones of equal amplitude. The tones are within the 3-dB bandwidth of the resonator. At the output, measured in the spectrum analyzer, we see the third-order mixing products resulting form the nonlinear impedance of the device under test. We measure the power in the intermodulation products as a function of the output power at the fundamental. At the bottom right is the typical plot of the measured data showing the fundamental as well as the third order intermodulation signal. Ground planes Intermod Output power (dBm) Used for measurement of ZS(Irf ,T, f), intermodulation distortion, and 3rd harmonic generation. Input power (dBm)

Dielectric Resonator for Films on Metallic Substrates Cross-section view of the dielectric resonator that is used to measure the microwave surface resistance of MgB2 films on metallic substrates. The stripline resonator cannot be used with conducting substrates. Can be used with metallic or dielectric substrates Designed for high-power measurements Fundamental frequency = 10.7 GHz TE011 mode

Resonators Stripline vs. Dielectric Stripline Resonator: Side View Dielectric Resonator: Top View rf magnetic field directions 150 mm Conductor Cross Section Cross section |J(x)/Jmax| Position from Center (m) |J(x)/Jmax| The differences in current distribution in the dielectric resonator and the stripline resonator are shown. These resonators have been used to compare ZS(Hrf) for films both patterned and unpatterned. The stripline resonator has a much higher and narrower peak than the dielectric resonator. Position from Center (mm) Jmax ~ 1.6 x 108 A/cm2 Jmax ~ 4.0 x 106 A/cm2

CW Measurement

CW Measurement Insertion loss (dB) Data Fit Frequency (Hz)

Time-Domain Pulsed Tests

Pulsed Measurement

Low Power Fit

Low-Power RS(T): MgB2 and Nb MgB2 on bulk Nb Dielectric resonator Niobium film on sapphire stripline MgB2 on sapphire Stripline resonator Low power surface resistance vs temperature for MgB2, and niobium. At 4 K the MgB2 on sapphire shows surface resistance comparable to or better that of niobium deposited at Lincoln Laboratory. This niobium is comparable to the best polycrystalline niobium films grown elsewhere. RS extrapolated to 2.2 GHz by f 2

RS vs HRF Dielectric and Metallic Substrates Dielectric res. Stripline res. Stripline res. Measurements of the power dependence of the surface resistance for films on various substrates in stripline and dielectric resonators. Also included are results for a niobium film. The films on sapphire and on niobium substrates are superior to the niobium film. The dielectric-resonator results are limited by the available power amplifier.

Breakdown Fields MgB2 on niobium substrate Measured in dielectric resonator T Max Pwr dBm QL (low pwr) Hrf max 4.2 28.7 7.8x106 278 7.5 38.2 5.5x106 697 11 No breakdown 2.5x106 470 Max power available Breakdown most likely due to thermal effects Amplifier with +45 dBm output power recently installed

Passivation Success at Film-Stabilization MgB2 Degrades in Air Passivation with 5 X (2.5 nm Al2O3 and 2.5 nm ZrO2) by ALD Over 6 Months & 5 Temperature Cycles Rs Unchanged. Q (f = 1. GHz)  1. 108 (Measured in a 2” Dielectric Resonator.) The graph shows the resonance curve measured in the dielectric resonator for a pair of wafers of MgB2 on a sapphire substrate with Al2O3 / Zro2 passivation deposited by atomic layer deposition. The Lorentzian fit yields a Q of 9.59 x 106. This is a Rs = 2.3 x 10-5 at 10.76 GHz, extrapolated by f2 to Rs = 2 x 10-7 at 1 GHz. RS(1 GHz ) = 2 x 10-7 Ω

Outline Introduction to MgB2 and Motivation Film deposition Measurements of surface impedance Physics of MgB2 from microwave measurements Summary

IMD and Rs vs Circulating Power MgB2 Resonator f1 f2 + Spectrum Analyzer Normalized IMD (dBm) Surface resistance (Ω) T = 20 K T = 2.5 K This shows the surface resistance and changes in surface reactance as a function of the circulating power at 2.5 and 20 K.. On the same graph referenced to the right hand scale is the measured IMD for the same temperatures. At the circulating power of interest for the IMD it is seen that the surface impedance measurements are not sensitive enough to resolve the nonlinearities. This illustrates the sensitivity of the IMD measurements. Circulating power (dBm) Similar plot for XS

IMD vs T: MgB2 and YBCO MgB2 samples t = 150 nm MgB2 t = 500 nm YBCO IMD vs T for YBCO and two typical MgB2 samples. Both show the characteristic 1/T2 dependence at low temperature. This is an indication of an order parameter with nodes giving rise to low-lying excitation. In YBCO it is d-wave symmetry and in MgB2 it is a six-fold symmetry compatible with the hexagonal crystal group. YBCO → d-wave order parameter → 1/T 2 at low T MgB2 → 6-fold symmetry → 1/T 2 at low T

Order Parameter - + ℓ = 6 MgB2 ℓ = 2 Cuprates On the left is the proposed order parameter for MgB2. This is the lowest order nodal symmetry allowed for a hexagonal crystal type. On the right for comparison is the accepted order parameter for the cuprates.

MgB2 is promising for applications Summary MgB2 is promising for applications Polycrystalline films with good RF properties Epitaxy not needed Low surface resistance and good power handling on dielectric and metallic substrates Microwave accelerator cavities – upgrade for niobium Remaining challenges Deposition on copper Conformal coating method Atomic layer deposition (ALD) Demonstrate Hrf > 2000 Oe Electronics applications on inexpensive substrates

Summary (Physics) Experimental evidence incompatible with s-wave symmetry Temperature dependence of intermodulation distortion (IMD) Increase of penetration depth at low temperatures Theory based on extension of constitutive relation (London theory) and ℓ = 6 symmetry of order parameter Nonlinear Meissner effect for IMD Andreev bound states for Dl(T)/l Conclusion: p gap has nodal order-parameter symmetry ℓ= 6 Implications for applications Linear low-T surface resistance Intermodulation distortion greater than s-wave

Low-T Penetration Depth and ℓ = 6 -Gap Symmetry This shows the fit of the theory to the data of the earlier slide of λ/λ vs temperature. The fit to the data is excellent. ● Good fits with plausible parameters for all sapphire cuts and for LAO 30

Nonlinear Meissner Effect Result of theory – nonlinear penetration depth Intrinsic Nonlinearity: From Pair Breaking by the RF Current IMD power Temperature dependence at low temperature For s-wave energy gap For energy gap with nodes This summarizes the nonlinear Meissner effect. We conclude from our measurements that MgB2 has unconventional symmetry of the energy gap. For MgB2, hexagonal symmetry rules out d-wave Best fit is with 6-fold symmetry D. Agassi and D. E. Oates, PRB 72, 014538 (2005), PRB 74, 024517 (2006) D. E. Oates, Y. D. Agassi and B. H. Moeckly, PRB 77, 214521 (2008) and Physica C 2012 in press

Resistive Transition Resistance vs temperature for a film on c-plane sapphire. The inset shows the region around Tc. The transition is very sharp and the resistivity low in agreement with other high-quality films.

YBCO on 5-cm diameter LaAlO3 Also dispersive-delay lines Long E-M Delay Lines YBCO on 5-cm diameter LaAlO3 44-ns delay Output Replace YBCO Inexpensive substrate No epitaxy Larger substrates for much longer delays 500 ns possible This is a photograph, extracted from the referenced publication, of a 44-ns delay line. This was implemented using YBCO. Such delay lines could be made less expensively and with far better yield using MgB2 because inexpensive substrates can be used and larger substrates can be used equally inexpensively for longer delays. Input Also dispersive-delay lines G. C. Liang et al. Trans MTT vol. 44, 1289, (1996)